GaN laminate and method of manufacturing the same

ABSTRACT

To provide a new GaN laminate obtained b growing a GaN layer on a GaN substrate by HVPE, including: a GaN substrate containing GaN single crystal and having a low index crystal plane as c-plane closest to a main surface; and a GaN layer epitaxially grown on the main surface of the GaN substrate wherein a surface of the GaN layer has a macro step-macro terrace structure in which a macro step and a macro terrace are alternately arranged, one of the macro step and the macro terrace has a step-terrace structure in which a step having a height of equal to or more than a plurality of molecular layers of GaN and extending in a direction orthogonal to m-axis direction, and a terrace are alternately arranged, and the other one of the macro step and the macro terrace has a step-terrace structure in which a step having a height of equal to or more than a plurality of molecular layers of GaN and extending in a direction orthogonal to a-axis direction, and a terrace are alternately arranged.

TECHNICAL FIELD

The present invention relates to a GaN laminate and a method ofmanufacturing the same.

DESCRIPTION OF RELATED ART

Gallium nitride (GaN) is used as a material for manufacturingsemiconductor devices such as light-emitting elements and transistors. AGaN laminate obtained by epitaxially growing a GaN layer on a GaNsubstrate is attracting attention because the GaN layer is of highquality (For example, see Non-Patent Document 1 for the application ofthe GaN substrate for high quality growth of the GaN layer).

For example, in order to improve a withstand voltage of a semiconductordevice manufactured using the GaN laminate, a thickness of the GaN layergrown on the GaN substrate is desired to be 10 μm or more. As atechnique for growing such a thick GaN layer on the GaN substrate, thepresent inventors propose to use hydride vapor phase epitaxy (HVPE)capable of obtaining a high growth rate compared to metalorganic vaporphase epitaxy (MOVPE) and the like.

[Non-Patent Document 1] Yuichi Oshima, five others, “GaN Substrate byVoid Formation Peeling Method”, Hitachi Cable Ltd. No. 26 (2007-1), p.31-36

It is not known very well hat kind of GaN laminate is obtained, bygrowing the GaN layer on the GaN substrate by HVPE.

An object of the present invention is to provide a new GaN laminateobtained by growing a GaN layer on a GaN substrate by and amanufacturing method for obtaining such a GaN laminate.

Summary of the Invention

According to an aspect of the present invention, there is provided a GaNlaminate including:

a GaN substrate containing GaN single crystal and having a low indexcrystal plane as c-plane closest to a main surface; and

a GaN layer epitaxially grown on the main surface of the GaN substrate,

wherein a surface of the GaN layer has a macro step-macro terracestructure in which a macro step and a macro terrace are alternatelyarranged,

one of the macro step and the macro terrace has a step-terrace structurein which a step having a height of equal to or more than a plurality ofmolecular layers of GaN and extending in a direction orthogonal tom-axis direction, and a terrace are alternately arranged, and

the other one of the macro step and the macro terrace has a step-terracestructure in which a step having a height of equal to or more than aplurality of molecular layers of GaN and extending in a directionorthogonal to a-axis direction, and a terrace are alternately arranged.

According to other aspect of the present invention, there is provided amethod of manufacturing a GaN laminate, including:

preparing a GaN substrate containing GaN single crystal and having a lowindex crystal plane as c-plane closest to a main surface; and

epitaxially growing a GaN layer on the main surface of the GaN substrateby HVPE,

wherein in epitaxially growing the GaN layer, the GaN layer is grown,with a growth temperature set to 950° C. or more and 1200° C. or less,and a macro step-macro terrace structure is formed on the surface of theGaN layer, in which a macro step and a macro terrace are alternatelyarranged,

one of the macro step and the macro terrace has a step-terrace structurein which a step having a height of equal to or more than a plurality ofmolecular layers of GaN and extending in a direction orthogonal tom-axis direction, and a terrace are alternately arranged, and

the other one of the macro step and the macro terrace has a step-terracestructure in which a step having a height of equal to or more than aplurality of molecular layers of GaN and extending in a directionorthogonal to a-axis direction, and a terrace are alternately arranged.

Advantage of the Invention

A new GaN laminate is provided. A method for obtaining such a GaNlaminate is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a laminate accordingto an embodiment of the present invention.

FIG. 2 is a flow chart schematically showing a method of manufacturingthe laminate according to an embodiment.

FIG. 3 is a schematic configuration view showing an HVPE apparatus.

FIG. 4A to 4E are views showing a surface state of an epi-layer of alaminate having a macro step-macro terrace according to an experimentalexample, respectively.

FIG. 5 is an enlarged view of FIG. 4A.

FIG. 6 is a schematic view of a macro step-macro terrace.

FIGS. 7A to 7C are views showing a surface state of the epi-layer in thelaminate produced by changing epitaxial growth conditions and substrateoff-angle conditions.

FIG. 8 is a graph showing first to third condition ranges in crystalgrowth treatment.

FIG. 9A and FIG. 9B are views showing a surface state of the epi-layeraccording to an experimental example, in a laminate produced using asubstrate in which off-direction of a central off-angle corresponds tom-direction.

FIG. 10A to FIG. 10C are views a surface state of the epi-layeraccording to an experimental example, in a laminate produced using asubstrate in which off-direction of a central off-angle corresponds tom-direction.

DETAILED DESCRIPTION OF THE INVENTION

A gallium nitride (GaN) laminate 30 according to an embodiment of thepresent invention will be described. FIG. 1 is a schematiccross-sectional view showing a GaN laminate 30. The GaN laminate 30(also referred to as a laminate 30) has a GaN substrate 10 (alsoreferred to as a substrate 10), and a GaN layer 20 (also referred to asan epi-layer 20) epitaxially grown on the substrate 10.

The substrate 10 contains GaN single crystal. The substrate 10 has amain surface 11, and the main surface 11 is a polished flat surface. Alow index crystal plane closest to the main surface 11 is a c-plane ofGaN single crystal constituting the substrate 10. Further, the c-planemay be a so-called +c-plane having a Ga polarity or a −c-plane having anitrogen polarity. An angle formed by c-axis direction at a certainposition in the main surface 11, and a normal direction of the mainsurface 11 (more specifically, a direction normal to the center of themain surface 11) is an off-angle of the substrate 10 at this position. Acentral off-angle, which is an off-angle at the center of the principalsurface 11, is formed, for example, so as to be inclined in a-axisdirection (also referred to as a-direction), and also formed, forexample, so as to be inclined in m-axis direction (also referred to asm-direction).

Inclination of the off-angle in the a-direction means that an angleformed by the a-direction and the off-direction which is the directionof the off-angle is less than ±15°, preferably within ±10°, morepreferably within ±5° in plan view of the substrate 10 (in plan view ofthe main surface 11). Similarly, inclination of the off-angle in them-direction means that an angle formed by the m-direction and theoff-direction which is the direction of the off-angle is less than ±15°,preferably within ±10°, more preferably within ±5° in plan view of themain surface 11. “The off-angle is inclined in a-direction” and “theoff-angle is inclined in m-direction” respectively are also referred toas “the off-direction corresponds to a-direction” and “the off-directioncorresponds to m-direction”.

The epitaxial layer 20 is a GaN layer epitaxially grown on the mainsurface 11 of the substrate 10. For example, in order to improve awithstand voltage of a semiconductor device manufactured using thelaminate 30, a thickness of the epi-layer 20 is preferably 10 μm ormore. In order to grow a thick epi-layer 20 of 10 μm or more at highspeed, hydride vapor phase epitaxy (HVPE) may be used as a growth methodof the epi-layer 20.

It is found by the inventors of the present invention that a uniqueridge-like structure is sometimes formed on the surface of the GaN layergrown on the GaN substrate by HVPE. More specifically, it is found thatsuch a ridge-like structure is more likely to be formed, when a thickGaN layer of 10 μm or more is grown in a predetermined condition range,for example, at a growth rate of 0.3 μm/min or more and 3.0 μm/minute orless. As will be described in detail later, in this specification, sucha unique ridge-like structure is referred to as a “macro step-macroterrace structure”. In order to avoid complicated description, the term“step-terrace structure” may be abbreviated as “step-terrace”.

It is unknown how the macro step-macro terrace affects a performance ofthe semiconductor device manufactured using the laminate 30. There is apossibility that the macro step-macro terrace may improve theperformance of the semiconductor device and there is also a possibilitythat the macro step-macro terrace may deteriorate the performance of thesemiconductor device. Therefore, as will be described in detail in theexperimental examples described later, the present inventors study on atechnique of promoting macro step-macro terrace formation, and atechnique of suppressing macro step-macro terrace formation.

Based on such a study, in this embodiment, explanation will be given forthe laminate 30 in which a macro step-macro terrace 40 is formed. Morespecifically, the laminate 30 according to the present embodiment has amacro step-macro terrace 40 on the surface 21 of the epi-layer 20.

As shown in FIG. 6 described later, the macro step-macro terrace 40 isinterpreted as a structure in which macro step 41 includingm-directional step-terrace 50 m, and macro terrace 42 includinga-direction step-terrace 50 a, are alternately arranged. It is presumedthat the m-directional step-terrace 50 m and the a-directionalstep-terrace 50 a have different various characteristics (For example,ease of incorporation of impurities affecting conductivity, and anincorporation ratio of group III atoms during growth of a mixed crystalsemiconductor such as InGaN and AlGaN etc.). Accordingly, the laminate30 having the macro step-macro terrace 40 also has potential to beapplied as a material for producing a new semiconductor device in whichfine regions having different characteristics (for example, differentconductivity and the like) are alternately arranged on the surface 21 ofthe epi-layer 20, for example, planar mixed crystal superlattice/dopedsuperlattice structure and one dimensional confinement structure such asquantum wire.

FIG. 2 is a flow chart schematically showing a method of manufacturingthe laminate 30. First, in step S10, the substrate 10 is prepared. Next,in step S20, the GaN layer 20 is epitaxially grown by HVPE on the mainsurface 11 of the substrate 10. Here, the HVPE apparatus 200 will bedescribed. FIG. 3 is a schematic configuration view showing the HVPEapparatus.

The HVPE apparatus 200 is formed from a heat-resistant material such asquartz, and includes a hermetic container 203, the inside of which isprovided with a film formation chamber 201. A susceptor 208 serving tohold the substrate 10 to be treated is provided inside the filmformation chamber 201. The susceptor 208 is connected to a rotary shaft215 of a rotary mechanism 216 and is configured to be rotatable. Gassupply pipes 232 a through 232 c serving to supply hydrochloric acid(HCl) gas, NH₃ gas, and nitrogen gas (N₂ gas) into the film formationchamber 201 are connected to one end of the hermetic container 203. Agas supply pipe 232 d serving to supply hydrogen (H₂) gas is connectedto the gas supply pipe 232 c. Flow rate control devices 241 a through241 d and valves 243 a through 243 d are provided respectively 011 thegas supply pipes 232 a through 232 d in that order from an upstreamside. A gas generation device 233 a that accommodates a Ga melt as a rawmaterial is provided on the downstream of the gas supply pipe 232 a. Anozzle 249 a is connected to the gas generation device 233 a. The nozzle249 a serves to supply gallium chloride (GaCl) gas produced by reactionbetween HCl gas and the Ga melt toward the substrate 10 held on thesusceptor 208. Nozzles 249 b and 249 c are connected respectively to thedownstream side of the gas supply pipes 232 b and 232 c. The nozzles 249b and 249 c serve to supply various gases supplied from the gas supplypipes 232 b and 232 c toward the substrate 10 held on the susceptor 208.A gas discharge tube 230 serving to discharge the gas inside the filmformation chamber 201 is provided on the other end of the hermeticcontainer 203. A pump 231 is provided on the gas discharge tube 230.Zone heaters 207 serving to heal the inside the gas generation device233 a and the substrate 10 held on the susceptor 208 to desiredtemperatures are provided around the outer periphery of the hermeticcontainer 203, and a temperature sensor 209 serving to measure thetemperature inside the film formation chamber 201 is provided in thehermetic container 203. The members included in the HVPE apparatus 200are connected to a controller 280 constituted as a computer and areconfigured such that processing procedures and processing conditionsdescribed later are controlled based on a program that is executed onthe controller 280.

The growth processing of the GaN layer 20 may, for example, beimplemented by the processing procedures below using the HVPE apparatus200. First, Ga is accommodated in the gas generation device 233 a as rawmaterial. The substrate 10 is held on the susceptor 208. Then, N₂ gas issupplied into the film formation chamber 201 while heating andexhausting the inside the film formation chamber 201, and the inside thefilm formation chamber 201 is set in a desired growth pressure. At thistime, in order to prevent roughening of the substrate surface beforestarting growth, supply of NH₃ gas is started from the gas supply pipe232 b, at a temperature of approximately 500° C. (preferably atemperature in a range of 400° C. to 550° C.). Then, after thetemperature of the inside the film formation chamber 201 reaches adesired growth temperature, supply of H₂ gas is started, and when theatmosphere in the film formation chamber 201 is in a desired atmosphere,gas is supplied from the gas supply pipe 232 a, thereby supplying GaClgas to the substrate 10. The reason why starting supply of the H₂ gasimmediately before supplying the GaCl gas is to prevent generation ofroughing due to applying etching to the surface of the GaN substratebefore growth when supply of the H₂ gas is started too early. It ispreferable to start supply of the H₂ gas within 2 minutes after thetemperature of the inside the film formation chamber 201 reaches adesired growth temperature, and it is preferable to supply GaCl gaswithin 1 minute after start of supply of the H₂ gas.

The following first condition range (also referred to as condition 1),second condition range (also referred to as condition 2), and thirdcondition range (also referred to as condition 3) are exemplified asgrowth conditions of the GaN layer 20. V/III ratio is the ratio of apartial pressure of the nitrogen (N) source gas (NH₃ gas in thisexample) which is a group V source gas, with respect to a partialpressure of a gallium (Ga) source gas (in this example, GaCl gas) whichis a group III source gas.

Growth conditions are exemplified as follows:

(V/III ratio)≤0.2 Tg−189 and (V/III ratio)≥0.2 Tg−199 as condition 1,

(V/III ratio)<0.2 Tg−199 and (V/III ratio)≥0.2 Tg−209 as condition 2,

(V/III ratio)<0.2 Tg−209 as condition 3,

in a range of growth temperature Tg: 950° C. or more and 1200° C., orless, V/III ratio: 1 or more and 51 or less, a pressure inside the filmformation chamber 201: 90 kPa or more and 105 kPa or less, preferably 90kPa or more and 95 kPa or less, partial pressure of GaCl gas: 0.3 kPa ormore and 1 5 kPa or less, and flow rate of N₂ gas/Flow rate of H₂ gas. 0or more and 20 or less.

FIG. 8 is a graph showing the range of growth temperature Tg and V/IIIratio corresponding to conditions 1, 2 and 3, in which a horizontal axisrepresents the growth temperature Tg, and a vertical axis represents theV/III ratio. In the condition range out of the conditions 1, 2 and 3 ofFIG. 8 (condition range indicated by gray), a surface roughness or adisorderly ridge-like structure, etc., which is different from theunique ridge-like structure of the present invention, are generated.

Preferably, condition 1 or condition 2 is used, and more preferablycondition 1 is used as the growth condition of the epi-layer 20 havingthe macro step-macro terrace 40. Namely, a relationship between theV/III ratio and the growth temperature Tg is preferably expressed as(V/III ratio)≤0.2 Tg−189 and (V/III ratio)≥0.2 Tg−209, and morepreferably expressed as (V/III ratio)≤0.2 Tg−189 and (V/III ratio)≥0.2Tg−199. The reason why condition 1 is more preferable is that the macrostep-macro terrace 40 is more likely to be formed, and the macrostep-macro terrace 40 can be formed irrespective of the off-angle of thesubstrate as will be described later, compared with condition 2. Thegrowth rate is, for example, in a range of 0.3 μm/minute or more and 3.0μm/minute or less.

Under condition 2, it is preferable to use the substrate 10 having aregion in which the off-angle size in the main surface 11 is preferably0.5° or less, more preferably 0.4° or less. This is because undercondition 2, the smaller the off-angle, the easier the formation of themacro step-macro terrace 40. In contrast, under condition 1, the macrostep-macro terrace 40 can be formed irrespective of the off-angle size.Under either condition 1 or condition 2, the macro step-macro terrace 40can be formed irrespective of the off-direction.

After growth of the GaN layer 20 having a predetermined thickness,preferably a thickness of, for example, 10 μm or more, supply of theGaCl gas used for the growth processing is stopped, and the temperatureinside the film formation chamber 201 is lowered to a temperature (nearroom temperature) at which an unloading operation can be performed. Inthis process, in order to protect the surface of epi-layer 20, it ispreferable that supply of the NH₃ gas is continued until the temperaturereaches about 500° C. (preferably at a temperature in a range of 400° C.or more and 550° C. or less). Further similarly, in order to protect thesurface of the epi-layer 20, supply of the H₂ gas is preferably stoppedat the same time as stopping supply of the GaCl gas. Then, afterreplacing the atmosphere inside the film formation chamber 201 with N₂gas to return to the atmospheric pressure, the substrate 10 with theepi-layer 20 formed thereon, that is, the laminate 30 is unloaded frominside the film formation chamber 201. In this way, the laminate 30having the macro step-macro terrace 40 on the surface 21 of theepi-layer 20 is manufactured.

Experimental Examples

Hereinafter, experimental examples will be described. In theexperimental examples, a laminate with the epi-layer having a thicknessof 10 μm or more grown on a substrate by HVPE is produced, and thesurface state of the epi-layer was checked. Resulting finding is asfollows: it is possible to separately form the laminate having the macrostep-macro terrace on the surface of the epi-layer, and form thelaminate having step-terrace which is flatter than the macro step-macroterrace on the surface of the epi-layer. The laminate of either of theseforms is also the GaN laminate having a new structure found by thepresent inventors.

In the experimental example, GaN single crystal substrate was used as asubstrate, the GaN single crystal substrate being manufactured byvoid-forming peeling (VAS) method and having a diameter of 2 inches(5.08 cm) and an in-plane threading dislocation density (TDD) of 1 to3×10⁶/cm². A plurality of substrates were used, each havingoff-direction and the central off-angle size different from each other.Specifically, substrates in which the off-direction of the centraloff-angle corresponds to a-direction, and substrates in which theoff-direction of the central off-angle corresponds to m-direction wereused, and substrates with the central off-angle size varied in a rangeof about 0.2° to about 0.6° were used. An epi-layer was formed on eachsubstrate by epitaxially growing GaN by HVPE, and a region in thevicinity of the center of the epi-layer was observed, whose off-angleand off-direction were already known. Since the VAS method is used, thesubstrate can be manufactured in order not to form a region where thethreading dislocation density is locally very high, for example, aregion having 1×10⁷/cm² or more. Therefore, it is preferable to use thesubstrate manufactured by the VAS method in order to enhance homogeneityin the plane of the epi-layer to be grown.

First, the macro step-macro terrace will be described, with reference toFIGS. 4A to 4E, FIG. 5 and FIG. 6. FIG. 4A to 4E are views showingsurface states of the epi-layer of the laminate having macro step-macroterrace. The laminate shown in FIG. 4A to FIG. 4E was manufactured bygrowing the epi-layer having a thickness of 30 μm under condition 1 onthe substrate in which the off-direction corresponds to a-direction andthe size of the central off-angle is 0.4°. FIG. 5 is an enlarged view ofFIG. 4A. FIG. 6 is a schematic view of the macro step-macro terrace.

FIG. 4A is an optical microscopic image of the epi-layer surface, FIG.4B is an atomic force microscope (AFM) image of a region of 20 μm squareon the epi-layer surface, FIG. 4C is an AFM image of a region of 5 μmsquare in the macro step, and FIG. 4E is an AFM line profile along aline shown in the center of FIG. 4B.

As shown in FIG. 4A, a unique ridge-like structure extending in anintermediate direction between a-direction and m-direction is observedon the epi-layer surface. Such a ridge-like structure was formed on theentire surface of the epi-layer. It was found by the present inventorsthat the ridge-like structure observed in FIG. 4A is schematicallyunderstood as the macro step-macro terrace 40 in which a macro step 41and a macro terrace 42 are alternately arranged, as shown in FIG. 6.Namely, it is found that the epi-layer surface of the laminate shown inFIGS. 4A to 4E is a surface having a macro step-macro terrace. The macrostep-macro terrace observed in FIG. 4A is a state of 90° rotation ofFIG. 6 in the page.

The direction and the interval of a striped pattern observed in the AFMimage are different, interposing a boundary area indicated as “macroterrace edge” in FIG. 4B The macro terrace edge corresponds to aboundary between the macro terrace 42 and the macro step 41 shown inFIG. 6.

FIG. 4C is an enlarged AFM image of the inside the macro step arrangedto the left of the macro terrace edge. A striped pattern with a narrowinterval (as compared with the image in FIG. 4D) is observed so as toextend in a direction approximately orthogonal to m-direction. As shownin FIG. 6, the macro step 41 is schematically understood as a surfacehaving a step-terrace structure in which m-directional step 51 m whichis the step extending in a direction orthogonal to m-direction, andin-directional terrace 52 m which is the terrace extending in adirection orthogonal to m-direction are alternately arranged. Further,the macro step 41 is schematically understood as a surface extending inthe intermediate direction between the a-direction and the m-direction.A step-terrace where the m-directional step 51 m and the m-directionalterrace 52 m are alternately arranged, is referred to as m-directionalstep-terrace 50 m. In FIG. 6, the m-directional step 51 m is indicatedby line.

The height of each m-directional step 51 m is, for example, about 1.2 to1.7 nm. Since the thickness of monolayer of GaN is about 0.26 nm, them-directional step 51 in is the step having a height of equal to or morethan a plurality of molecular layers of GaN, for example, a height ofabout 4 to 7 layers of GaN, and is the step in which step bunchingoccurs in the m-direction.

FIG. 4D is an enlarged AFM image of the inside the macro terracearranged to the right of the macro terrace edge, and a striped patternwith a wide interval (as compared with the image in FIG. 4C) is observedso as to extend in a direction approximately orthogonal to thea-direction corresponding to the off-direction. As shown in FIG. 6, themacro terrace 42 is schematically understood as a surface having astep-terrace in which a-directional step 51 a which is the stepextending in a direction orthogonal to the a-direction, anda-directional terrace 52 a which is the terrace extending in a directionorthogonal to the a-direction, are alternately arranged. Further, themacro terrace 42 is schematically understood as a surface extending inthe intermediate direction between the a-direction and the m-direction.The step-terrace where the a-directional step 51 a and the a-directionalterrace 52 a are alternately arranged, is referred to as a-directionalstep-terrace 50 a. In FIG. 6, the a-directional step 51 a is indicatedby line.

The height of each m-directional step 51 a is, for example, about 1.2 to2.4 nm. Accordingly, the a-directional step 51 a has a height of equalto or more than a plurality of molecular layers of GaN, for example, aheight of about 4 to 10 layers, and is the step in which step bunchingoccurs in the m-direction.

FIG. 4E shows a macro step-macro terrace line profile straddling themacro step on the left and the macro terrace on the right interposingthe macro terrace edge shown in FIG. 4B. The profile shows that astepped structure is formed in the macro step-macro terrace. Although itis difficult to understand from the line profile of FIG. 4E, as shown inFIG. 6, an inclination direction of the macro step 41 and an inclinationdirection of the macro terrace 42 with respect to a reference surface 22of the epi-layer (the surface being defined as a surface with leveledunevenness of the macro step-macro terrace 40) are different from eachother. A surface having the macro step 41, namely, the m-directionalstep-terrace 50 m is a surface inclined by 0.5° or more and 0.7° or lesswith respect to the c-plane. A height of the peak-to-valley ofunevenness formed by the macro step 41 and the macro terrace 42 is 10 nmor more.

Contour lines shown in FIG. 5 and FIG. 6 are lines connecting them-directional step 51 m of the macro step 41 and the a-directional step51 a of the macro terrace 42. In a plan view of the epi-layer (in planview of the reference surface 22), it is presumed that a directionorthogonal to the direction in which the contour lines extend averagelyis an actual off-direction. Therefore, the actual off-direction in theobserved region is slightly deviated from a correct a-direction.

A relationship between epitaxial growth conditions and ease of macrostep-macro terrace formation, and a relationship between substrateoff-angle conditions and ease of macro step-macro terrace formation willbe described next, with reference to FIG. 7A to FIG. 7C. FIG. 7A to FIG.7C show a surface state of the epi-layer in the laminate produced bychanging the epitaxial growth conditions and the substrate off-angleconditions.

A plurality of laminates shown in FIGS. 7A to 7C were produced asfollows. Three types of substrates were used as substrates, in which thecentral off-angle sizes are 0.25°, 0.4° and 0.55°, and theoff-directions of the central off-angles correspond to the a-direction,respectively. The laminate was produced by growing the epi-layer with athickness of 30 μm on each kind of the above substrates, under pluralgrowth conditions of conditions 1, 2, and 3. From FIG. 8, it is foundthat conditions 1, 2 and 3 are the same as the above-describedconditions. Substantially the same results were obtained within eachcondition range of the conditions 1, 2 and 3, and therefore FIG. 7A toFIG. 7C show typical results in each condition range.

FIG. 7A shows an optical microscopic image of the epi-layer surface ofeach laminate together, FIG. 7B shows an AFM image of a region of 5 μmsquare on the epilayer surface of the laminate having the off-angle of0.25° under a growth condition of condition 3, and FIG. 7C shows an AFMimage of a region of 5 μm square on the epi-layer surface of thelaminate having the off-angle of 0.4° under a growth condition ofcondition 3.

As shown in FIG. 7A, under condition 1, macro step-macro terrace isformed on the entire surface of the epi-layer regardless of anyoff-angle size. Under condition 2, the smaller the off angle, the easierit is to form the macro step-macro terrace, and when the off angle is0.55°, almost no macro step-macro terrace is formed. This result revealsthat under condition 2, the off-angle of the substrate is preferably0.5° or less, more preferably 0.4° or less in order to form the macrostep-macro terrace.

Under condition 3, regardless of any off-angle size, the macrostep-macro terrace is not observed, and a flat epi-layer surface can beobtained compared to a case where the macro step-terrace is formed.

As shown in FIG. 7B, in the laminate having the off-angle of 0.25° undercondition 3, a uniform striped pattern is observed in the AFM image.This shows that the epi-layer surface has the step-terrace in which astep and a terrace extending in a predetermined direction (fixeddirection) are alternately arranged, and namely shows that the epi-layersurface is not a surface having two types of step-terrace (m-directionalstep-terrace and a-directional step-terrace) like a macro step-macroterrace, but a surface having one type of step-terrace. An opticalmicroscopic image shown in FIG. 7A reveals the following matter: aregion of at least 1 mm square or more on the epi-layer surface isconstituted by one type of step-terrace, and similarly an opticalmicroscopic image shown in FIG. 7C reveals the following matter, also inthe laminate having the off-angle of 0.4° under condition 3, a uniformstriped pattern is observed in the AFM image, and a region of at least 1mm square or more on the epi-layer surface is constituted by one type ofstep-terrace.

In the laminate (laminate having an off-angle of 0.25°) shown in FIG. 7Band the laminate (laminate having an off-angle of 0.4°) shown in FIG.7C, the epi-layer surface has a step-terrace in which a step having aheight of equal to or more than a plurality of molecular layers of GaNand extending in a predetermined direction, and a terrace arealternately arranged. However, there are the following differencesbetween the step-terrace of the laminate having the off-angle of 0.25°and the step-terrace of the laminate having the off-angle of 0.4°. Thestep of the laminate having the off-angle of 0.25° is a double step inwhich each step has a constant height of two molecular layers of GaN. Incontrast, the step of the laminate having the off-angle of 0.4 is thestep in which step bunching occurs, and having a height of approximatelyabout 4 to 10 GaN layers in the a-direction. The step of the laminatehaving the off-angle of 0.4° extends in a direction approximatelyorthogonal to the a-direction corresponding to the off-direction. Theflat epi-layer surface as compared with the macro step-macro terraceformation is schematically understood as a surface having such astep-terrace.

A case where the off-direction of the substrate corresponds to them-direction will be described, with reference to FIG. 9A, FIG. 9B, andFIG. 10A to FIG. 10C. FIG. 9A, FIG. 9B, and FIG. 10A to FIG. 10C areviews showing a surface state of the epi-layer in the laminate producedusing a substrate in which the off-direction of the central off-anglecorresponds to the m-direction. The laminate shown in FIGS. 9A and 9B(laminate having the off-angle of 0.3°) was produced by growing theepi-layer with a thickness of 30 μm under condition 1, on the substratehaving the central off-angle size of 0.3° in the off-directioncorresponding to the m-direction. The laminate shown in FIG. 10A to FIG.10C (laminate having the off-angle of 0.55°) was produced by growing theepi-layer with a thickness of 30 μm under condition 2, on the substratehaving the central off-angle size of 0.55° in the off-directioncorresponding to the m-direction.

FIG. 9A is an optical microscopic image of the epi-layer surface in thelaminate having the off-angle of 0.3°, and FIG. 9B is an AFM image of aregion of 20 μm square on the epi-layer surface in the laminate havingthe off-angle of 0.3°.

As shown in FIG. 9A, the macro step-macro terrace is observed on theepi-layer surface. As shown in FIG. 9B, the macro step is observed onthe right side and the macro terrace is observed on the left sideinterposing the macro terrace edge which is a boundary area. In themacro step, the m-directional step extending in the directionapproximately orthogonal to the m-direction corresponding to theoff-direction and having a narrow interval is observed, and in the macroterrace, the a-directional step extending in the direction approximatelyorthogonal to the a-direction and having a wide interval is observed. Asdescribed above, also in the case where the off-direction of thesubstrate corresponds to the m-direction, the macro step-macro terracecan be formed on the epi-layer surface, as in the case where theoff-direction of the substrate corresponds to the a-direction.

FIG. 10A is an optical microscopic image of the epi-layer surface in thelaminate having the off-angle of 0.55°, FIG. 10B is an AFM image of aregion of 20 μm square on the epi-layer surface in the laminate havingthe off angle of 0.55°, and FIG. 10C is an AFM image of a region of 5 μmsquare on the epi-layer surface in the laminate having the off angle of0.55°.

As shown in FIG. 10A, the epi-layer surface is a flat surface having nomacro step-macro terrace observed. As shown in FIG. 10B and FIG. 10C, auniform striped pattern is observed in the AFM image, and it is foundthat the epi-layer surface is the surface having one type ofstep-terrace. From the optical microscopic image shown in FIG. 10A, itcan be said that a region of at least 1 mm square or more on theepi-layer surface is constituted by one type of step-terrace. Theobserved step extend in a direction approximately orthogonal to them-direction corresponding to the off-direction. Further, the step has aheight of equal to or more than a plurality of molecular layers of GaN,and in the step, step bunching occurs in the m-direction. As describedabove, also in the case where the off-direction of the substratecorresponds to the m-direction, a flat epi-layer surface can be obtainedcompared with the case where the macro step-macro terrace is formed, asin the case where the off-direction of the substrate corresponds to thea-direction. Namely, the epi-layer surface can be formed not as asurface having two types of step-terrace (m-directionalstep/m-directional terrace and a-directional step/a-directional terrace)like the macro step-macro terrace, but as a surface having one type ofstep-terrace.

Regarding the formation of such a Hat epi-layer, the following can besaid. When comparing the laminate having the off-angle of 0.25° shown inFIG. 7B, the laminate having the off angle of 0.4° shown in FIG. 7C, andthe laminate having the off-angle of 0.55° shown in FIG. 10C, it ispresumed as follows: the off-angle of the substrate becomes largeexceeding 0.3°, thereby increasing the force to align the extendingdirection of the step in the a-direction or in the direction orthogonalto the m-direction corresponding to the off-direction. Namely, in orderto form the step-terrace so that the step extend in the directionorthogonal to the a-direction or the m-direction corresponding to theoff-direction, it may be preferable to set the off-angle of thesubstrate to more than 0.3°.

Further, since the off-angle of the substrate becomes as small as 0.3°or less, a double step in which each step has a constant height of twomolecular layers of GaN are likely to be formed, although a detailedreason is unknown. Namely, in order to form the step-terrace having thedouble step, it may be preferable to set the off-angle of the substrateto 0.3° or less.

As described above, it is possible to separately form the laminatehaving the macro step-macro terrace on the epi-layer surface, and thelaminate having the step-terrace which is flatter than the macrostep-macro terrace on the epi-layer surface. There is a tendency suchthat the lower the growth temperature, the easier the formation of theformer laminate, and the higher the growth temperature, the easier theformation of the latter laminate. The reason therefore is presumed to beas follows.

As shown in FIG. 6, in order to form the macro step-macro terrace, atomsconstituting GaN are required to diffuse to a predetermined positionwhere the macro step-macro terrace is formed. As the temperature ishigher, the atoms easily evaporate from the epi-layer surface, sosurface diffusion hardly occurs. Therefore, it is considered that themacro step-macro terrace is less likely to be formed as the temperaturebecomes high, and the epi-layer surface becomes flat.

Actually observed macro step-macro terrace (see for example FIG. 4B andFIG. 9B), or actually observed step-terrace (see for example FIG. 7B,FIG. 7C and FIG. 10C) include fluctuations, and therefore thesestructures sometimes don't perfectly match the structure based on theabove-described schematic understanding. However, based on theabove-described schematic understanding, it is possible to grasp thecharacteristics of each of the macro step-macro terrace and thestep-terrace.

Other Embodiments

As described above, embodiments of the present invention have beenspecifically described. However, the present invention is not limited tothe above-described embodiments, and various modifications,improvements, combinations, and the like can be made without departingfrom the gist of the present invention.

In the above-described explanation, for the convenience of explanation,the macro step-macro terrace is described such that the surfaceconstituted by the m-directional step-terrace is referred to as themacro step, and the surface constituted by the a-directionalstep-terrace is referred to as the macro terrace. Experimental examples(see FIGS. 4B and 9B) reveal a tendency such that the step interval(terrace width) in the macro step (m-directional step-terrace) and thestep interval (terrace width) in the macro terrace (a-directionalstep-terrace) are different from each other, and more specificallyreveal a tendency such that the step interval in the macro step(m-directional step-terrace) is narrower than the step interval in themacro terrace (a-directional step-terrace). However, depending onvarious conditions, there is a possibility that the step interval in themacro step (m-directional step-terrace) is wider than the step intervalin the macro terrace (a-directional step-terrace). Further, depending onthe off-angle and off-direction of the substrate, it is also presumedthat the step direction of the macro step and the macro terrace isreversed. Namely, it is also possible to form a macro step-macro terracehaving a macro terrace constituted by the m-directional step-terrace,and a macro step constituted by the a-directional step-terrace. In eachease, the macro step-macro terrace is similarly constituted as acombination of two types of step-terrace as described above. In thesurfaces of the macro step-macro terrace, each being constituted by twotypes of step-terrace, namely, in the macro step and the macro terrace,a wider one may be referred to as “a macro terrace”, and a narrower onemay be referred to as “a macro step” for convenience.

The laminate having the macro step-macro terrace on the epi-layersurface is not limited to the laminate having the macro step-macroterrace on the entire surface of the epi-layer, and may be the laminatehaving the macro step-macro terrace in a part of the epi-layer surface(for example, a region having a size of 1 mm square or more or a regionhaving a width of 500 μm square or more).

The laminate having the step-terrace which is flatter than the macrostep-macro terrace on the epi-layer surface is not limited to thelaminate having the step-terrace on the entire surface of the epi-layer,and may be the laminate having the step-terrace in a part of theepi-layer surface (for example, a region having a size of 1 mm square ormore or a region having a width of 500 μm square or more).

Impurities such as conductivity type determination impurities may beincluded in each of the substrate and the epi-layer of the laminate. Theepi-layer may have one of the GaN layer containing n-type impurities andthe GaN layer containing p-type impurities, or may have both.

Preferable Aspects of the Present Invention

Preferable aspects of the present invention will be supplementarydescribed hereafter.

(Supplementary Description 1)

There is provided a GaN laminate, including:

a GaN substrate containing GaN single crystal and having a low indexcrystal plane as c-plane closest to a main surface; and

a GaN layer epitaxially grown on the main surface of the GaN substrate,

wherein a surface of the GaN layer has a macro step-macro terracestructure in which a macro step and a macro terrace are alternatelyarranged,

one of the macro step and the macro terrace has a step-terrace structurein which a step having a height of equal to or more than a plurality ofmolecular layers of GaN and extending in a direction orthogonal tom-axis direction, and a terrace are alternately arranged, and

the other one of the macro step and the macro terrace has a step-terracestructure in which a step having a height of equal to or more than aplurality of molecular layers of GaN and extending in a directionorthogonal to a-axis direction, and a terrace are alternately arranged.

(Supplementary Description 2)

There is provided the GaN laminate according to supplementarydescription 1, wherein the macro step and the macro terrace extend in anintermediate direction between m-axis direction and a-axis direction.

(Supplementary Description 3)

There is provided the GaN laminate according to supplementarydescription 1 or 2, wherein a step interval in the macro step and a stepinterval in the macro terrace are different from each other.

(Supplementary Description 4)

There is provided the GaN laminate according to any one of thesupplementary descriptions 1 to 3, wherein the step interval in themacro step is narrower than the step interval in the macro terrace.

(Supplementary Description 5)

There is provided the GaN laminate according to any one of thesupplementary descriptions 1 to 4, wherein the macro step is a surfaceinclined by 0.5° or more and 0.7° or less with respect to c-plane, themacro step having a step-terrace structure in which a step having aheight of equal to or more than a plurality of molecular layers of GaNand extending in a direction orthogonal to m-axis direction, and aterrace are alternately arranged.

(Supplementary Description 6)

There is provided the GaN laminate according to any one of thesupplementary descriptions 1 to 5, wherein a height of a peak-to-valleyof an unevenness formed by the macro step and the macro terrace is 10 nmor more.

(Supplementary Description 7) There is provided the GaN laminateaccording to any one of the supplementary descriptions 1 to 6, wherein athickness of the GaN layer is 10 μm or more.

(Supplementary Description 8)

There is provided the GaN laminate according to any one of thesupplementary descriptions 1 to 7, wherein a region having a size of 1mm square or more on a surface of the GaN layer has the macro step-macroterrace structure.

(Supplementary Description 9)

There is provided the GaN laminate according to any one of thesupplementary descriptions 1 to 8, wherein the substrate is thesubstrate manufactured by a VAS method (It is the substrate notincluding a region having a threading dislocation density of 1×10⁷/cm²or more).

(Supplementary Description 10)

There is provided a method of manufacturing a GaN laminate, including:

preparing a GaN substrate containing GaN single crystal and having a lowindex crystal plane as c-plane closest to a main surface; and

eptaxially growing a GaN layer by HVPE on the main surface of the GaNsubstrate,

wherein in epitaxially growing the GaN layer, the GaN layer is grown,with a growth temperature set to 950° C. or more and 1200° C. or less,and a macro step-macro terrace structure is formed on the surface of theGaN layer, in which a macro step and a macro terrace are alternatelyarranged,

one of the macro step and the macro terrace has a step-terrace structurein which a step having a height of equal to or more than a plurality ofmolecular layers of GaN and extending in a direction orthogonal tom-axis direction, and a terrace are alternately arranged, and

the other one of the macro step and the macro terrace has a step-terracestructure in which a step having a height of equal to or more than aplurality of molecular layers of GaN and extending in a directionorthogonal to a-axis direction, and a terrace are alternately arranged.

(Supplementary Description 11)

There is provided the method of manufacturing a GaN laminate accordingto supplementary description 10, wherein in epitaxially growing the GaNlayer, a range of growth temperature Tg and V/III ratio is set to (V/IIIratio)≤0.2 Tg−189 and (V/III ratio)≥0.2 Tg−209.

(Supplementary Description 12)

There is provided the method of manufacturing a GaN laminate accordingto supplementary description 10, wherein in epitaxially growing the GaNlayer, a range of growth temperature Tg and V/III ratio is set to (V/(IIratio)≤0.2 Tg−189 and (V/III ratio)≥0.2 Tg−199.

(Supplementary Description 13)

There is provided the method of manufacturing a GaN laminate accordingto supplementary description 10, wherein in epitaxially growing the GaNlayer, a range of growth temperature Tg and V/III ratio is set to (V/IIIratio)<0.2 Tg−199 and (V/III ratio)≥0.2 Tg−209.

(Supplementary Description 14)

There is provided the method of manufacturing a GaN laminate accordingto supplementary description 13, wherein the substrate has a region inthe main surface, in which a size of an off-angle is preferably 0.5° orless, more preferably 0.4° or less, the off-angle being the angle formedby a normal direction of the main surface and c-axis direction.

(Supplementary Description 15)

There is provided the method of manufacturing a GaN laminate accordingto any one of the supplementary descriptions 10 to 14, wherein inepitaxially growing the GaN layer, the GaN layer having a thickness of10 μm or more is grown.

(Supplementary Description 16)

There is provided the method of manufacturing a GaN laminate accordingto any one of the supplementary descriptions 10 to 15, wherein inepitaxially growing the GaN layer, the GaN layer is grown at a growthrate of 0.3 μm/minute or more and 3.0 μm/minute or less.

(Supplementary Description 17)

There is provided the method of manufacturing a GaN laminate accordingto any one of the supplementary descriptions 10 to 16, wherein inepitaxially growing the GaN layer, after supply of the N source gas,supply of hydrogen gas is started, and after supply of the hydrogen gas,supply of Ga source gas is started.

DESCRIPTION OF SIGNS AND NUMERALS

10 GaN substrate (substrate)

11 Main surface of the (substrate 10)

20 GaN layer (epi-layer)

21 Surface of the (epi-layer)

30 GaN laminate (laminate)

40 Macro step-macro terrace structure

41 Macro step

42 Macro terrace

50 m m-directional step-terrace structure

51 m m-directional step

52 m m-directional terrace

50 a a-directional step-terrace structure

51 a a-directional step

52 a a-directional terrace

200 HVPE apparatus

The invention claimed is:
 1. A GaN laminate, comprising: a GaN substratecontaining GaN single crystal and having a low index crystal plane asc-plane closest to a main surface; and a GaN layer epitaxially grown onthe main surface of the GaN substrate, wherein a surface of the GaNlayer has a macro step-macro terrace structure in which a macro step anda macro terrace are alternately arranged, one of the macro step and themacro terrace has a step-terrace structure in which a step having aheight of equal to or more than a plurality of molecular layers of GaNand extending in a direction orthogonal to in-axis direction, and aterrace are alternately arranged, and the other one of the macro stepand the macro terrace has a step-terrace structure in which a stephaving a height of equal to or more than a plurality of molecular layersof GaN and extending in a direction orthogonal to a-axis direction, anda terrace are alternately arranged.
 2. The GaN laminate according toclaim 1, wherein the macro step and the macro terrace extend in anintermediate direction between m-axis direction and a-axis direction. 3.The GaN laminate according to claim 1, wherein a step interval in themacro step and a step interval in the macro terrace are different fromeach other.
 4. The GaN laminate according to claim 1, wherein the macrostep is a surface inclined by 0.5° or more and 0.7° or less with respectto c-plane, the macro step having a step-terrace structure in which astep having a height of equal to or more than a plurality of molecularlayers of GaN and extending in a direction orthogonal to m-axisdirection, and a terrace are alternately arranged.
 5. The GaN laminateaccording to claim 1, wherein a height of a peak-to-valley of anunevenness formed by the macro step and the macro terrace is 10 nm ormore.
 6. A method of manufacturing a GaN laminate, comprising: preparinga GaN substrate containing GaN single crystal and having a low indexcrystal plane as c-plane closest to a main surface; and eptaxiallygrowing a GaN layer by HVPE on the main surface of the GaN substrate,wherein in epitaxially growing the GaN layer, the GaN layer is grown,with a growth temperature set to 950° C. or more and 1200° C. or less,and a macro step-macro terrace structure is formed on the surface of theGaN layer, in which a macro step and a macro terrace are alternatelyarranged, the macro step has a step-terrace structure in which a stephaving a height of equal to or more than a plurality of molecular layersof GaN and extending in a direction orthogonal to m-axis direction, anda terrace are alternately arranged, and the macro terrace has astep-terrace structure in which a step having a height of equal to ormore than a plurality of molecular layers of GaN and extending in adirection orthogonal to a-axis direction, and a terrace are alternatelyarranged.
 7. The method of manufacturing a GaN laminate according toclaim 6, wherein in epitaxially growing the GaN layer, a range of growthtemperature Tg and V/III ratio is set to (V/III ratio)≤0.2 Tg−180 and(V/III ratio)>0.2 Tg−209.
 8. The method of manufacturing a GaN laminateaccording claim 6, wherein in epitaxially growing the GaN layer, a rangeof growth temperature Tg and V/III ratio is set to (V/III ratio)≤0.2Tg−189 and (V/III ratio)≥0.2 Tg−199.
 9. The method of manufacturing aGaN laminate according to claim 6, wherein in epitaxially growing theGaN layer, a range of growth temperature Tg and V/III ratio is set to(V/III ratio)<0.2 Tg−199 and (V/III ratio)≥0.2 Tg−209.
 10. The method ofmanufacturing a GaN laminate according to claim 9, wherein the substratehas a region in the main surface, in which a size of an off-angle is0.5° or less, the off-angle being the angle formed by a normal directionof the main surface and c-axis direction.